1. Field of the Invention
This invention relates to an improved plasma flame spray powder, and to a method for producing such powder, and to composite articles of manufacture including outer plasma flame sprayed coatings of such powders, and more particularly relates to a cermet plasma flame spray powder, method for producing it, and articles therefrom.
2. Prior Art
Free flowing powders are useful in a variety of applications in the ceramic and metallurgical arts, such as in the formation of powder compacts, in casting and in coating operations, such as plasma flame spraying.
Metallic and ceramic flame spray coatings are frequently applied to various articles to impart properties such as hardness, wear resistance, good lubricity, corrosion resistance, improved electrical properties or perhaps simply to build up a used part which has worn below usable tolerances.
The concept of combining metals and ceramics is widely used today to achieve various superior properties of the resulting composite material which could not be achieved by use of either component alone. Cermets offer the combination of strength and toughness of metals with the high temperature resistance of ceramics. This combination also provides unique properties in plasma flame sprayed coatings. For example, coatings of superior wear resistance can be produced with cermet compositions.
One difficulty encountered in the prior art with cermet flame spray powders has been the temperature resistance of the ceramic component. It would be desirable to melt both components completely in the short residence time in the plasma flame in order to achieve a dense, well bonded flame sprayed coating.
Prior art includes several methods for combining metal and ceramic components in flame spray powder form. The simplest is to mix individual powders, each of a size generally acceptable for plasma flame spraying, about minus 100 mesh to plus 10 microns. However, because of the difference in melting points of the metal and ceramic powders and the individual reaction of each component particle in the plasma flame, it is difficult to melt both components without overheating, the lower melting component, resulting in undesirable oxidation of the metallic component or in extreme cases its vaporization during spraying.
Another approach is to produce individually clad particles. Either a core of metal clad with a ceramic or a core of ceramic clad with metal is proposed in U.S. Pat. No. 3,655,425 by Longo et al. The choice of materials for combination is usually limited in this method, however, due to the limited ability to clad one material upon another, particularly in view of the desire to retain some exposure of the surface of the core material for best results.